There is a need for medical devices that can be placed in tubular organs of the human or mammalian body that are self expanding and can stay in place. These medical devices can serve a number of purposes. For example, they can act as a stent to maintain the body lumen, or they can act to hold or anchor other devices such as filters or indwelling catheters. The use of stents to maintain the patency of bodily lumens is well known. Stents are typically delivered in an unexpanded configuration via a catheter to a desired bodily location. Once at the desired bodily location, the stent is expanded and implanted in the bodily lumen. The stent may self-expand or may be mechanically expanded.
Self-expanding stents or medical devices are fabricated larger than the size of the lumen by some amount so that, once implanted into the lumen, the stent exerts some amount of radially outward force on the lumen as it seeks to return to its as-fabricated configuration. Self-expanding devices are subject to high stresses when the diameter is reduced down onto the catheter for insertion into the body; these stresses are largely relieved when the stent is released inside the body lumen. The delivery means such as a delivery catheter for a self-expanding device must be capable of holding the stent in this high stress configuration until it releases the device inside the body lumen. This is usually accomplished by retention devices on the delivery means such as a retractable sheath, retention wires, clips, or other mechanical means capable of resisting the forces due to the internal stress in the device. The delivery means must be capable of releasing the device in a controlled way such that the desired final position of the stent within the body lumen is accurately maintained.
Devices that are mechanically expanded are fabricated to be just slightly larger than the delivery means, and after insertion into the body cavity are subject to large forces that plastically deform the device to a new size inside the body lumen. The force may be applied via an expandable member such as a balloon or via any other mechanical device.
Devices are used in an array of hollow body cavities including arteries and veins, such as the coronary arteries, the peripheral arteries, arteries of the neck, and cerebral arteries. They may also be used in non-blood contacting spaces such as, biliary ducts, urethras, ureters, fallopian tubes, bronchial tubes, the trachea, the esophagus, and within the digestive tract and the prostate.
For devices used in the peripheral vascular system, the physiological deformations exerted by the body on the device are substantially different from those experienced by a stent in the coronary arterial system. For example, in the arteries of the leg, specifically the superficial femoral artery, there are substantial axial, torsional, and flexing deformations that have no counterpart in the coronary arterial system. This means that the same device designs that work in the coronary arteries are not necessarily going to work optimally in the arteries of the leg.
Previous groups have used coil-based stents or devices with limited clinical success. The purpose of the claimed invention is to address and correct the problems with previous generations of coil-based devices. Prior generations of coil stents fall into three groups. The first group consists of stents that are composed of a single fiber wound in an unbroken helical coil of uniform diameter and even spacing (that is, the number of turns of the coil/cm is a constant value). The second group is a rather unique design, again consisting of a single fiber wound into a coil-based design, but where the winding sense of the coil changes direction (right hand thread to left hand thread) at regular intervals. Finally, the third group (and clinically the most successful to date) is composed of multiple fibers, each of which is individually a helical coil, yet they are woven or braided together in such a way as to make a highly flexible, closed cell design.
The following discussion is directed to the first two groups mentioned above. The specific clinical problems encountered to date by various devices within these two groups of coil-based devices are listed below:                a. Tissue prolapse between the coils, i.e. insufficient coverage of the vessel wall        b. High rates of restenosis        c. Difficult to deliver, i.e. long delivery times and typically awkward delivery catheters        d. Inaccurate placement due to dramatic jumping of the stent upon delivery, and/or different lengths of the stent on and off the catheter.        e. Stent migration post deployment        f. Limited to short stent lengths        g. Collapse of the stent under shear load        h. Limited to constant diameter stents        
All coil-based devices should be designed to reduce restenosis, have high biocompatibility, and a low profile. Further, all such devices must be accurately placed and easy to deliver. In addition to these general requirements, devices used in the arteries of the leg also require mechanical compliance in axial extension and compression, flexion and torsion while maintaining radial stiffness.
The design criteria for a device appropriate for peripheral vascular circulation, and the solution arrived at, forms the basis for the claimed invention. The invention is a helical coil design consisting of multiple reversing sense helical coil units, that are capable of drug elution, come in lengths appropriate for long, diffuse lesions, have the ability to have a step-wise tapering diameter, and provide all the benefits of a small closed cell design while maintaining high flexibility, high radial force and crush resistance due to the underlying helical coil.
Although the claimed invention is appropriate for the peripheral vascular system, the invention itself is much broader in scope and can be applied to a large number of medical devices. Any medical device that is required to maintain position within any tubular anatomical structure can benefit from this invention.